The present invention relates to ultrasonic sensors for measuring physical properties of materials within a defined space, and relates more particularly to a time gate ultrasonic sensor system for very high temperatures and pressures.
Various ultrasonic distance and level measuring sensors are known. For example, ultrasonic sensors have been used as point level switches for detecting high and low material levels in a process container. Industrial process applications include petroleum, chemical, pulp and paper, mining and materials, oil and gas applications, among others. In these applications, a point level switch is often the last means of detecting an overflow or underflow condition. Safety, measurement accuracy, and sensor integrity are critical.
Esin et al. U.S. Pat. No. 5,269,188 discloses a continuous self-test time gate ultrasonic sensor system which measures presence or density of a process material within a defined space. The sensor includes a transmit transducer and receive transducer which are connected to opposite sides of a U-shaped support structure for transmitting and receiving ultrasonic signals across the defined space. The ultrasonic signals include a main waveform which travels across the defined space and a self-test waveform which travels along the support structure. The sensor senses the physical property of the material within the defined space as a function of whether the main waveform is received within the main time window. The sensor performs a self-test during each measurement cycle by sensing whether the self-test waveform is received during a self-test time window which is different from the main time window. For a correctly operating sensor, the self-test waveform should be present during each measurement cycle.
A deficiency of this type of ultrasonic sensor is that the transmit and receive transducers are bonded to the inside of the sensor body which is submerged in the process to be measured. For piezoceramic transducers, for example, the maximum temperature of the process in which this sensor can be used is therefore limited to a temperature that is below the Curie point of the transducers to avoid depolarization and other damage to the transducers or their bonds to the support structure. In many applications, the process to be measured is hazardous, such as with flammable or explosive materials, and may be under very high temperatures and pressures. An ultrasonic sensor capable of operating under very high temperature and pressures is desired.